Velocity Voxel Scale is a new parameter, introduced in Houdini 19.5. The parameter allows for separate definitions for volume and and velocity voxels in pyro simulations. Depending on the parameter settings, you will observe a significant change in simulation time.
Velocity Voxel Scale is available for DOP and SOP pyro networks.
For DOP setups, the Velocity Voxel Scale parameter is on the Smoke Object (Sparse) node under Properties.
For SOP setups, you can find the settings in the Pyro Solver under Setup ▸ General.
Velocity Voxel Scale is a multiplier and applied on top of the Voxel Scale parameter. Higher values create a coarser grid with bigger cells. This results in faster simulations, but you will also lose detail on the motion. So you could have Voxel Size at
0.1 and Velocity Voxel Scale at
2 to get a simulation where every field uses
0.1, except velocity will be at
0.1*2=0.2. This way, you have lots of voxels in the visible fields so they look sharp, but fewer velocity voxels where a lot of the expensive calculations happen. The caveat is that you have fewer degrees of freedom in the velocity. The motion will be less detailed compared to doing a simulation at full resolution.
To give you some figures and facts on simulation times and results, please take a look at the images.
The parameter combination is Voxel Scale x Velocity Voxel Scale. Please note that the given time factors are just exemplaric and can’t be seen as benchmarks.
The first simulation uses
0.1 x 1. Both grid scales are equal. Simulation time is set as the baseline (1.0).
The second parameter combination is
0.1 x 2. The cell size of the velocity grid is two times bigger. The simulation was 1.6× faster.
In the third image, scales are
0.2 x 1. Again, both grid scales are equal, but compared to the first simulation, cooking time was 7.5× faster.